In the manufacture of corrugated board sheets, there has been a persistent problem of producing these sheets without warpage. Although there have been several attempts to solve this problem none have done so satisfactorily in either reducing and preventing the warpage or, where the board can be flattened, in a manner which is economically acceptable.
An explanation of the types of warp or curvature in the corrugated board as well as some of the devices which have characterized previous attempts to overcome this problem will be described in connection with FIGS. 1 through 5. This should enhance an understanding of the invention described herein and yield an appreciation of the deficiencies in the prior art.
In referring to FIGS. 1 and 2 there are shown the types of undesirable curvature which are often imparted to corrugated board during the manufacture process. In FIG. 1, for example, curves are produced transverse to the path of movement of the board through the corrugating machinery as indicated by the arrows. These curves, when viewed from the top as shown in FIG. 1, can be concave as in 1(a), convex as in 1(b) or a combination of the two, producing an S-curve as shown in 1(c). Curvature can be produced in the direction of movement of the board which is concave as shown in 2(a), convex as shown in 2(b).
The warp or curvature produced in the direction of movement of the board through the machine, such as that shown in FIG. 2, is often attributable to the tension or stress placed on the board as it moves through the rollers comprising the machine. This type of curvature is primarily eliminated through adjustment of the rollers and mechanisms within the machine to compensate for the undesirable stress or tension otherwise produced.
On the other hand, the curvature transverse to the direction of movement of the board through the machine is primarily caused by an unbalance in the moisture content in the various parts of the board. For example, in connection with FIG. 3, it can be seen that a liner 4 is drawn or pulled through double facer 2 to interface with a single faced board 3 being fed through the same system. The various components of the double facer 2 press and heat the single faced board 3 and the liner 4 previously treated with glue to secure these elements together and form the composite corrugated board in a known manner. Where there is an imbalance in the moisture content of the single faced board 3 compared with the liner 4 warpage in a direction transverse to the direction of movement of the board can occur. For example, the concave curvature of FIG. 1(a) occurs when the amount of moisture of the liner of single faced board 3 is greater than that in the liner of the lower sheet 4 shortly before the single faced board 3 and the liner 4 are glued together in the double facer 2. After the gluing step the moisture content of both liners are balanced by drying. Because of the initial moisture imbalance, one side may dry more quickly than the other and, with the accompanying unequal shrinkage, a curve is obtained. Conversely, where the moisture in the liner 4 is greater than that of the single faced board 3 before they are glued together, the convex curve will occur as shown in FIG. 1(b). The S-curvature of 1(c) results from a combining of the relative moisture contents between the single faced board 3 and the liner 4 where one side of board liner 4 has a greater moisture content than single faced board 3 and the other side the liner 4 has a lower moisture content than single faced board 3. In any event, it has not been possible to control the warp in the direction transverse to the movement of the corrugated board in a satisfactory manner.
An example of previous attempts to solve the warpage problem is shown in FIG. 4. There it can be seen that the corrugated board sheets 1 formed in double facer 2, shown in a larger view in FIG. 4, are piled by a stacker 5. Because of the difficulty in accurately determining the moisture content of liners during continuous operation at a high speed within the corrugating machine, an operator checks the condition of warp sheets 6 which are piled up by the stacker 5. When warpage is detected, this is corrected through the operation of control panel 7. Specifically, to obtain an acceptably flat sheet, the operator controls the moisture content of the corrugated board accumulated in a stack by the stacker by regulating shower portions 8a and 8b of the single facer, and warp angle adjustment for the preheater rolls, 9a, 9b, 9c, 9d, and 9e, and through the handling of control ballast roll 11 of double facer 2, as well as the showers 10a, 10b, and 10c of the input portion of the double facer 2. This type of apparatus requires significant discretion on the operator and consequently, requires a high degree of skill. Furthermore, even if the sheets in the stacker were delivered and piled flatly, warpage would still occur because the corrugated sheets often had an imbalance of moisture.
Another attempt to correct warpage is to restack the warped sheets in an alternating manner as shown in batch 12 of FIG. 5. To accomplish this, after the sheets have been cut and stacked automatically in the normal manner from the manufacturing machine, an operator is made available to rearrange the pile such that alternate sheets are overturned. In this manner each sheet has a warp or curve in a direction substantially opposite to the sheets on either side of it. Because of this alternating disposition of sheets the dead weight of the stack will tend to correct the warp. This alternate stacking operation requires a great deal of manpower not only to correct warpage but also to restack the sheets in preparation for printing or box making operations in subsequent processing. Thus, although this approach may satisfactorily achieve a flat disposition prior to further processing, it has been uneconomical, due to the exceptional manpower demands, in correcting the warpage in the corrugated board manufacturing.
The invention described herein overcomes many of the problems described above by achieving a flat disposition for the sheets cut and ejected from the manufacturing machinery in an efficient and economical manner; particularly, the manpower restraints which have characterized some attempts in solving this problem are substantially overcome. In summary, this is accomplished by actually imparting differing warps at certain intervals on the sheets in a specified manner so that, as they are stacked after being cut, the dead weight of the sheets in the stack will eliminate the warpage in a satisfactory manner. This approach avoids the high degree of accuracy and skill required by an operator attempting to balance moisture applied to upper and lower liners. Because the warp is varied in a particular manner prior to the sheets being stacked, there is no need for operator rearranging the sheets after they have been stacked in preparation for further printing or other operations. Rather, the sheets as delivered are in the proper disposition for overcoming and correcting the warp as well as further printing and processing.
More specifically, the warp imparted to the sheets during the manufacturing process is accomplished, in one form of the invention, by spraying the upper liners and lower liners in an alternate fashion. For example, at given intervals the upper liner is sprayed while the lower liner is untreated; after that interval has passed the system is reversed whereby the lower liner is sprayed while the upper liner remains untreated. In this way the corrugated board is warped in different directions during each interval of spray. The board is subsequently cut and automatically stacked. As a result of the alternating spray system, the stacked corrugated boards will correspondingly alternate in warp throughout the height of the stack. The dead weight of the stack will cause these alternating warps to flatten and produce ultimately an acceptably flat sheet.
As will be explained further in the detailed description of the preferred embodiment, a control system is integrated with the fabricating machinery to automatically sense a length of run of the board as it passes through the machine and to activate and deactivate upper and lower spray apparatus for alternating the spray as described above. A preselected interval is obtained by relating the interval of spray to the length of board passing through properly placed sensing apparatus.
The amount of spray directed to either the upper or lower liners over a given interval can be adjusted to control the moisture content for the board. A selection mechanism is provided where the automatic controller can activate, of the series of spray nozzles on a given side of the board, only those selected while leaving the unselected spray nozzles unactivated throughout the entire process. This prevents those portions of the liner where spraying is not required from being sprayed under the automatic control system. Thus, the operator, when dealing with the board as it comes off the fabricating machine, can regulate the amount of moisture being imparted to the board through selection and adjusting mechanisms provided in the automatic controller.
These features which overcome many of the problems confronted by the prior art as well as other features of the invention will be better appreciated in the description of the preferred embodiment which follows hereinafter.